Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project

Abstract: We present Spitzer follow-up imaging of 95 candidate extremely cold brown dwarfs discovered by the Backyard Worlds: Planet 9 citizen science project, which uses visually perceived motion in multi-epoch WISE images to identify previously unrecognized substellar neighbors to the Sun. We measure Spitzer [3.6]-[4.5] color to phototype our brown dwarf candidates, with an emphasis on pinpointing the coldest and closest Y dwarfs within our sample. The combination of WISE and Spitzer astrometry provides quantitative confirmation of the transverse motion of 75 of our discoveries. Nine of our motion-confirmed objects have best-fit linear motions larger than 1"/yr; our fastest-moving discovery is WISEA J155349.96+693355.2 (total motion ~2.15"/yr), a possible T type subdwarf. We also report a newly discovered wide-separation (~400 AU) T8 comoving companion to the white dwarf LSPM J0055+5948 (the fourth such system to be found), plus a candidate late T companion to the white dwarf LSR J0002+6357 at 5.5' projected separation (~8,700 AU if associated). Among our motion-confirmed targets, five have Spitzer colors most consistent with spectral type Y. Four of these five have exceptionally red Spitzer colors suggesting types of Y1 or later, adding considerably to the small sample of known objects in this especially valuable low-temperature regime. Our Y dwarf candidates begin bridging the gap between the bulk of the Y dwarf population and the coldest known brown dwarf.

• The paper confirms extremely cold brown dwarfs using Spitzer photometry and WISE-Spitzer astrometry, identifying 75 motion-confirmed targets.
• The paper’s photometric analysis reveals five Y-dwarf candidates with red Spitzer colors, including four likely of Y1 or later type.
• The paper uncovers new wide-separation companions to white dwarfs, highlighting the potential of citizen science to enhance the substellar census.

Spitzer Follow-up of Extremely Cold Brown Dwarfs

The study delineated herein investigates extremely cold brown dwarfs identified through the Backyard Worlds: Planet 9 citizen science project, leveraging Spitzer follow-up imaging to substantiate these findings. The primary goal was to assort brown dwarf candidates by measuring their Spitzer [3.6]-[4.5] color, thereby focusing on Y dwarfs, the coldest substellar neighbors to the Sun. A sample of 95 brown dwarf candidates underwent Spitzer photometric characterization. Crucial confirmations of motion were achieved for 75 targets through the synthesis of WISE and Spitzer astrometric datasets.

Summary of Findings

1. Motion Confirmation and Photometric Analysis:
   • Among the motion-confirmed objects, nine exhibited linear motions exceeding 1''/yr, with WISEA J155349.96+693355.2 marked as a fast mover at approximately 2.15''/yr.
   • The photometric analysis identified five objects with Spitzer colors most consistent with spectral type Y, with four of these displaying notably red colors suggestive of Y1 or later, thus expanding the known low-temperature brown dwarf sample.
2. Notable Companion Discoveries:
   • A significant find includes a newly identified wide-separation (∼400 AU) T8 comoving companion to the white dwarf LSPM J0055+5948. Another discovery was a potential late T companion to the white dwarf LSR J0002+6357 at a projected separation of ∼8,700 AU.
3. Significance in Solar Neighborhood Studies:
   • These discoveries underscore the capability of the Backyard Worlds project and its citizen science framework in enhancing the census of substellar objects in proximity to the solar system.

Implications and Future Directions

The research sets a critical foundation for future spectroscopic characterizations by the James Webb Space Telescope (JWST), aiming at the high-priority targets within the low mass and temperature regime analogous to isolated exoplanets. The constantly improving identification and characterization techniques entail profound implications for understanding the Galactic substellar population's spatial distribution and composition. This paper prompts further probes into the dynamics of motion and characteristics of substellar objects within the solar neighborhood, potentially aiding in refining models of brown dwarf formation and evolution. These advances also carry the potential to uncover more about objects residing along the lighter spectrum of the mass function, influencing our broader understanding of star and planet formation paradigms.